Which solar panels work best in the shade?

When trees or buildings cast partial shadows on a solar panel, it causes power to drop dramatically. Some types of panels are designed to better handle this.

Photo of solar panels under a cloudy sky
Flickr

Low light conditions are a normal situation that solar panels are well designed to handle.

Whether it’s cloudy, rainy, or simply early in the morning or late in the afternoon, you should expect your solar panels to continue to generate power. The power output of your solar panels will drop, but with most panels the decrease is fairly linear. That is, if the sunlight drops by half, with most panels the output will also drop by half.

However, a funny situation happens when a solar panel is only partially covered by a shadow. If you block light from half of a panel and leave the other half in bright sunlight, you might expect that the power output will drop by half. But that’s not the case. What actually happens is that the power output will drop dramatically - you will lose much more than half the power.

There are two approaches that manufacturers take to build solar panels that work better in partial shade conditions like this. The first, and most common, is to use an electronic component called a bypass diode. The purpose of a bypass diode is to allow electricity in a solar panel to flow around the shaded cell. A second approach is to use half-cut solar cells. The use of half-cut solar cells increases the available electric pathways in a solar panel, making it more resistant to shade.

A visual explanation of how shading affects solar panels

To help you understand exactly what is meant by partial shading and how it affects the power output of a solar panel, check out this video from the altE store. It’s only five minutes long, but explains the issue really well.

In the video, she shows that the drop in current changes depending on where the shadow falls. This is because the individual cells are wired in series, and where the shadow falls in the series will affect how much current is blocked.

A typical home solar panel has 60 cells wired in series. Here’s a simplied diagram, where the red line represents the wiring that connects the cells:

Diagram of how solar cells are wired together in a panel.

Shaded cells block the flow of electricity, so you can imagine that a shaded cell near the end of the string will be worse than a shadow near the start of it.

Bypass diodes in solar panels

Exactly how much power a solar panel will lose when partially shaded depends on where shadow falls on the panel. A typical solar panel is made of 60 individual solar cells wired together. When a solar cell is shaded, it loses power. That loss of power increases electrical resistance within the cell, which in turn blocks the flow of electricity coming from upstream solar cells. Depending on where the shadow falls, a solar panel without a method to migitate this problem could lose power entirely.

The solution that manufacturers use to address this type of partial shading issue is to add bypass diodes. A bypass diode is like a detour around a traffic jam: it allows electricity to flow around shaded cells. The use of bypass diodes in solar panels is required by the IEC 61215 testing standard.

For a more detailed explanation about how bypass diodes work, check out this article at Electronics Hub.

A typical 60 cell solar panel is internally wired into three separate strings, with one bypass diode protecting each string. These diodes sit in the junction box, which is the little black box you’ll find on the back of the panel. With a bypass diode in place, when an individual solar cell is shaded, power loss occurs only within the same string, not the entire panel. This can limit the impact of partial shading on the panel, but it does depend on how the shadow falls on the panel.

Here’s another version of the diagram above, modified to illustrate the wiring of three bypass diodes:

Illustration showing the effect of shading on a solar panel with bypass diodes



The bypass diodes are marked in green, and you can see that each diode is wired into a string of 20 cells (60 cells in the panel, divided by three).

In this illustration, there’s a shadow that falls against one edge of the panel. Without bypass diodes, that partial shadow could effectively shut down the entire panel. But with the addition of bypass diodes, the diode allows electricity to flow around the high electrical resistance caused by the shaded cells, allowing the remaining 2/3rds of the panel to continue to operate.

But if the shadow falls in a different orientation, it will cause at least a partial power loss for the entire panel:

Illustration showing the effect of shading on a solar panel when the shadow falls across multiple strings



In a home rooftop installation, shading can come from many sources, not just trees. Neighboring buildings, chimneys, rooftop objects like satellite dishes, differently angled segments of the roof, and even leaves and bird poop can cause shading problems. While all solar panels approved for home use in the United States will have bypass diodes, some manufacturers have additional methods to improve performance in this situation.

One of these technologies is half-cut cells, which basically doubles the ability of a solar panel to deal with shading issues.

Half-cut solar cells

A half-cut solar cell is exactly what it sounds like: a normal solar cell is cut in two. A panel that uses half-cut cells will have 120 half-cut cells instead of the typical 60.

With the wiring scheme of a half-cut solar panel, this results in a doubling of electrical pathways through the panel. This means that a panel will have six internal strings rather than three.

Diagram of a solar panel with half-cut solar cells. (Courtesy REC Solar)
Diagram of a solar panel with half-cut solar cells. (Courtesy REC Solar)

As you can see in the diagram above, a solar panel with half-cut cells has two regions, doubling the electrical pathways through the panel. If the top half of the panel is shaded, the bottom will be unaffected.

Lower resistance in half-cut cells means better efficiency

One of the ways that a solar panel loses efficiency is through heat caused by electrical resistance. An electrical system isn’t a perfect conductor. The next time you use a hair dryer, feel the cord when you’re done. It’ll be warm, and that’s because of electrical resistance in the wire, which causes some of the electricity to be wasted as heat.

The same thing happens in a solar panel. The equation for power loss is this:

power loss = current² × resistance

This means that if you double the current (measured in amps), the power loss is 4 times higher. Conversely, if you reduce the current by half, the power loss drops by a factor of 4.

Because of this, half-cut cells benefit from lower resistive power loss. Each half-cut solar cell generates half as much current as a full sized cell, resulting in 1/4th the power loss of a conventional panel.

How much more efficient is a half-cut solar panel?

Resistive power loss within a panel isn’t very high, but we’ve reach the point in solar panel technology where every little gain helps.

On average, a half-cut solar panel will have an absolute efficiency improvement of about 0.5%. This means that if you have a panel with 18% overall efficiency, switching to half-cut cells will get you to about 18.5% efficiency.

Disadvantages of half-cut cells

The main disadvantage of half-cut cells is somewhat higher costs due to increased manufacturing complexity. There is additional wiring and a second junction box, which increases material costs.

Also, half-cut cells are made by making normal size solar cells and cutting them in half. That process of splitting the cells results in a small percentage of breakage, increasing manufacturing waste.

Using half-cut cells requires more wiring and solder points, resulting in greater opportunities for wiring failures to occur. It’s too early to tell, but it’s possible that panels with half-cut cells may have higher failure rates.

Because of this, it might be a good idea to pay particular attention to the product warranty when going with half-cut solar panels. Of the products listed below, the REC TwinPeak has the best warranty with 20 year product coverage, and a 25 year power warranty. Many other panels have only a 10 year product warranty.

Manufacturers making panels with half-cut cells

Despite the tradeoff of higher manufacturing complexity, with companies fighting to gain an advantage in a very competitive marketplace, more and more manufacturers are using half-cut cells.

One another benefit is that half-cut cells can be combined with other technologies to improve cell efficiency even further. For example, you can find PERC, heterojunction and bifacial solar panels that use half-cut cells.

This is a partial list of manufacturers and their products that feature half-cut cells:

The industry is fast-moving, so ask your installer if their preferred manufacturer has a product line featuring half-cut cells.

Side note: American made half-cut solar panels coming

If you’re interested in buying American-made solar panels, you might want to know that Hanwha (a South Korean company) is building a manufacturing plant in Georgia and will be assembling their Q.Peak DUO G6 panel there.

The website states that assembly will be done in Georgia, but it doesn’t say where the actual cells will be manufactured. That information might become more clear when the product hits the market. In any case, this product is something to keep an eye on if buying American is something that’s important to you.

Are half cut cells worth it?

Half-cut cells are just one approach to addressing partial shading conditions and improving solar panel efficiency. Whether it’s worth it really depends on your roof conditions, which is something you should discuss in detail with your installer.

If you have shading conditions only for part of the day, then the cost of a premium panel might just not be worth it. Efficiency is only one consideration when comparing panels, and many times the higher price of a premium panel won’t be recouped by its increased efficiency. Read my article on finding the best solar panels to learn more about how to weigh the different characteristics of solar panels when deciding which one to buy.

Microinverters, power optimizers, and shading

To be clear, this article is about partial shading of a single solar panel, and not shading that affects several solar panels in your array.

Shade that falls across part of your array causes a system-wide problem that is similar to the partial shading of a single solar panel. This is because, just like how individual cells in a panel are wired in series, the panels in an array are also normally wired in series. This means blocking the light on a single panel can affect the entire array.

The solution to this lies with your inverter choice. Microinverters and power optimizers are two inverter technologies that allow your array to better deal with system shading. Both will help maximize your system power output, with the tradeoff of higher cost.

To learn more about inverter choices, read my guide to how solar inverters work.



TAGS:
#Panel Technology #Panel Efficiency

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